Powdered wax, or paraffin powder, is used in commerce in large amounts for a host of different applications some of which require that the particles of wax be solid, essentially spherical and in the size range from about 1 .mu. (micron) to about 177 .mu. or even larger. The term "wax" as used herein refers not only to paraffin waxes but to wax-like materials which have the physical characteristics of paraffin wax and are used in generally analogous applications. By "essentially spherical" we mean that there is less than 1% deviation between any two axes detectable under a magnification of 75.times. (seventy five times).
For example large wax beads in the range from about 1 mm to about 15 mm are not essentially spherical, and do not need to be because they are typically used in decorative applications, particularly for "bead curtains" and in mock-ups of jewelry featuring pearls and beads of amber, amethyst, jade, etc. of different colors and sizes. Relatively smaller wax beads, over a wide range of sizes from 44 .mu. to about 1 mm may be injection molded into a desired shape to be used as a pattern from which a shell mold is to be made for an investment casting. A few applications, such as for printing inks and varnish compositions, require a narrower size range of particles in the range from about 1 .mu. to about 44 .mu., more preferably limited to a range of less than about 20 .mu., but with no specific regard for sphericity. This is also true in those instances where finely divided solid wax particles are used as inert fillers in tablets of pharmaceuticals.
In neither of the prior art applications is there an emphasis on sphericity because there is no special need for such a property. If the particles made were essentially spherical it was not because sphericity was particularly sought. For the particular purpose to which the invention is directed, namely, forming a sintered body as described below, it is essential that individual wax particles be essentially spherical so that the force required to roll a first mass of particles over another is minimal, and it is a particular characteristic of the claimed process that the microspheres made in a two-tier weight distribution described below, are essentially spherical. An added benefit of having essentially spherical microspheres in the twotier weight distribution is that it provides the sintered body with better surface definition.
U.S. Pat. No. 4,898,486 to Meck et al (class 400/subclass 241.2) describes the use of powdered wax in a melt-transfer color layer for a thermal printing head, e.g. of the dot-matrix type. U.S. Pat. No. 4,846,887 to Kuehnle (class 106/subclass 31) teaches a process for making micronized wax by spraying a wax melt to obtain a powder of spherical particles, then grinding the powder.
Depending upon the size of the wax spheres desired, many different methods have been used to make them. For example, a wax melt is sprayed into water through one or more nozzles so as to produce a liquid stream of wax which is broken up, one way or another, into a multiplicity of droplets. To obtain relatively large wax spheres the melt may be introduced under pressure, into a temperature-controlled water bath, either downwardly as in U.S. Pat. No. 2,570,423 to Batchelder et al, or, upwardly as in U.S. Pat. No. 4,384,835 to Bland ( both in class 425/subclass 10).
Instead of spraying the molten wax into water or other liquid in which the wax is immiscible, molten wax under pressure is sprayed into air through a nozzle, either upwardly or downwardly. These methods of spraying molten wax into a gas stream generally produce smaller wax spheres than methods in which the molten wax is sprayed onto or into a liquid. For example, wax spheres in the range from about 18 .mu. to about 80 .mu. are formed by spraying upwardly into a controlled high-velocity stream of cold air, as more fully described in U.S. Pat. No. 3,868,199 (class 425/subclass 10) to Fera. The spheres formed are graded into four groups, namely, those over 80 .mu., those under 18 .mu., those between 45 .mu.-90 .mu., and those between 18 .mu.-45.mu.. Thus, under any conditions, more than half the spheres by weight (wt), are always smaller than 18 .mu.; less than 5% by wt are over 80 .mu.. It is evident that the foregoing particular distribution of sizes is of wax spheres which are small (hence referred to as "microspheres") and were tailored to find specific use in applications which demanded the particular size distribution Fera made. In most cases, as in his, it is the specific proposed purpose for which the microspheres are to be used which determines the size ranges in which they are to be made. Whatever his purpose, it is evident he did not teach making and using microspheres having a size distribution such that more than half (&gt;50%) the cumulative weight percent is attributable to particles having a diameter greater than a predetermined diameter (175 .mu., in particular). This distribution of wax microspheres by our process is referred to as a "two-tier weight distribution".
A particular distribution of sizes of wax microspheres is of especial interest, and in particular demand for a method to produce a shaped article by a selective laser sintering (SLS) process disclosed in U.S. Pat. Nos. 4,863,538 to Deckard; 4,938,816 to Beaman et al; and, 4,944,817 to Bourell et al, the disclosure of each of which is incorporated by reference thereto as if fully set forth herein. The shaped article is formed by sintering a powder of one or more materials. "Sintering" is defined by the heating of the powder to a temperature which causes viscous flow only at contiguous boundaries of its particles, with at least some portion of substantially all particles remaining solid. Such sintering causes coalescence of particles into a sintered solid mass, the density of which is increased compared to the bulk density of the powder particles before they were sintered; and, a part formed by layer-wise joining of plural vertically contiguous layers is therefore said to be autogenously densified.
The goal of the SLS process using a wax powder as the feed material, is to produce a solid porous article from the powder, which article not only has the precise dimensions of the shape desired, but is dense enough to provide a pattern from which a shell mold is to be made. The denser (that is, less porous and smaller void fraction) the article, the stronger it is, hence the denser the better. It is essential that the wax powder feed be readily flowable and easily pourable at an elevated temperature below the initial melting point of the wax, under applied force only sufficient to spread the powder over a target area in a SLS machine in which the SLS process is practiced. To provide the wax powder with such fluidity, shapes other than essentially spherical, are inadequate and ineffective for the purpose at hand.
To our knowledge, there has never been a need for a two-tier weight distribution of wax particles produced by a process which directly generated a mass of microspheres such that more than half (&gt;50%) the cumulative weight percent is attributable to particles having a diameter greater than a predetermined diameter (175 .mu. is most preferred for the task at hand) for the particular purpose of packing at least some, and preferably a major portion of the interstitial spaces between the larger particles with the smaller ones. Therefore there has been no motivation for one to address such a specific problem.
For the reason given, we did, and in a very simple process, directly produced not only the desired size range of wax microspheres by a novel process, but produced them in a desirable, substantially two-tier weight distribution of sizes. When the process is used to generate microspheres essentially none which are smaller than about 125 .mu. the process does not meet the requirements of the specific SLS purpose for which the microspheres are to be used. Moreover, such smaller particles are formed in too narrow a size range, as will be evident from the data presented as a graph in FIG. 7.
Because the SLS process determines the size range and wax microspheres, a brief description of the SLS process follows.
The SLS process is carried out in an apparatus which includes a laser or other directed energy source which is selected and tailored to emit a beam of desired intensity in a target area where a three-dimensional wax part is produced. A powder dispenser system deposits powder into the target area. A laser control mechanism operates to move the aim of the laser beam and modulates the laser to selectively sinter only the wax powder disposed within defined boundaries to produce a two-dimensional portion (layer) of the part. The control mechanism operates selectively to sinter sequential layers of powder each within the defined boundaries, producing a completed part comprising a plurality of layers sintered together. The defined boundaries of each layer correspond to respective cross-sectional regions of the part. Preferably, the control mechanism includes a computer--e.g. a CAD/CAM system to determine the defined boundaries for each layer. That is, given the overall dimensions and configuration of the part, the computer determines the defined boundaries for each layer and operates the laser control mechanism in accordance with the defined boundaries for each layer. Alternatively, the computer can be initially programmed with the defined boundaries for each layer.
A part is produced by depositing a first portion of powder onto a target surface, scanning the aim of a directed energy beam (preferably a laser) over the target surface, and sintering a first layer of the first powder portion on the target surface. The first layer corresponds to a first cross-sectional region of the part. The powder is sintered by operating the directed energy source when the aim of the beam is within the boundaries defining the first layers. A second portion of powder is deposited onto the first sintered layer and the aim of the laser beam scanned over the first sintered layer. A second layer of the second powdered portion is sintered by operating the directed energy source when the aim of the beam is within the boundaries defining the second layer. Sintering of the second layer also joins the first and second layers into a cohesive mass. Successive portions of powder are deposited onto the previously sintered layers, each layer being sintered in turn.
Repetition of the foregoing steps results in the formation of a bed of powder which continually presents the target surface, and if the wax particles of powder are overheated by the beam at the boundaries of the article, the sharp definition of the boundaries is lost. It is therefore essential that the wax particles of powder outside the boundaries of the article to be formed, withstand being sintered and retain their individual particulate identities.
Considering that the sintered body is to be made as dense as possible without sacrificing the sharp definition of the boundaries of the article, we came to realize that the feed powder should be of relatively large wax microspheres (referred to as "large spheres"), and that the interstitial spaces between a mass of such spheres be filled with much smaller microspheres (referred to as "small spheres"). For example, knowing that spheres packed in cubic, body centered cubic and face centered cubic packing configurations have void fractions of 0.48, 0.32 and 0.26, respectively, one can estimate that randomly packing a unit volume with microspheres of identical diameter will result in a void fraction of about 0.33 (that is, leaves a 0.33 fraction of void space between and around the microspheres). Because there is a geometrical relationship between the diameter of a large microsphere and of a smaller one just large enough to fit in the interstitial spaces between packed large microspheres, we sought to produce a mass of microspheres which would be a very rough approximation of a mixture of spheres having basically only two ranges of diameters, each as narrow as possible, namely, a large diameter range for "large" spheres, and a smaller diameter range for "small" spheres.
Of course, one could produce precisely the correct mixture of sizes by producing a mass of wax spheres in a wide range of diameters, then sieving, or somehow painstakingly separating the spheres into various fractions, and mixing only the desired fractions. Except that it is impractical to separate microspheres smaller than about 25 .mu. with sieves. Moreover, only relatively large wax spheres, larger than about 44 .mu. lend themselves to sieving, the smaller sieves being soon "blinded" by very quickly. Further, though there are separation methods using differences in the velocity of wax spheres of different sizes falling through a liquid, such methods are impractical on a commercial scale. At the present time we know of no practical method for separating large wax microspheres in a range in which the average large diameter is greater than about 175 .mu., from small wax microspheres in which the average small diameter is less than one-half the average large diameter.
In practice, large spheres cannot be made within as narrow a range as one would desire, but only in a relatively broad range of diameters, so that the interstitial spaces between four contiguous spheres would not be the same, and neither would the diameter of the small microspheres required to fit in those interstitial spaces. But there was no reason to believe that one could not make "large" spheres within a relatively narrow range of diameters. Such "large" spheres would in turn, require "small" spheres (for the interstitial spaces) also within a relatively narrow range. Thus, it would be highly desirable to practice a process which would, for the most part, produce a product having a two-tier weight distribution.
We have discovered a novel and very simple process to produce a two-tier distribution of microspheres which when sintered, yield such a product.